819 line

Analog high-definition television was an analog video broadcast television system developed in the 1930s to replace early experimental systems with as few as 12-lines. On 2 November 1936 the BBC began transmitting the world's first public regular analog high-definition television service from the Victorian Alexandra Palace in north London. It therefore claims to be the birthplace of television broadcasting as we know it today. John Logie Baird, Philo T. Farnsworth, and Vladimir Zworykin had each developed competing TV systems, but resolution was not the issue that separated their substantially different technologies, it was patent interference lawsuits and deployment issues given the tumultuous financial climate of the late 1920s and 1930s.

Most patents were expiring by the end of World War II leaving no worldwide standard for television. The standards introduced in the early 1950s stayed for over half a century.

When the UK introduced 405-line television broadcasting in 1936, it was described as 'high definition television'. By today's standards it most certainly was not even approaching high definition. The description merely referred to its definition in comparison to the early 30-line (largely) experimental system broadcast in the 1920s.

When Europe resumed TV transmissions after WWII (i.e. in the late 1940s and early 1950s) most countries standardized on a 576i (625-line) television system. The two exceptions were the British 405-line system, which had already been introduced in 1936, and the French 819-line system developed by René Barthélemy. During the 1940s Barthélemy reached 1015-lines and even 1042-lines. On November 20, 1948, François Mitterrand, the then Secretary of State for Information, decreed a broadcast standard of 819-lines; broadcasting began at the end of 1949 in this definition.

...
Wikipedia